The present invention is directed to a self-cure mold process for preparing foundry sand molds which utilizes vinyl acetate resin as binder for the sand-containing mold or core, bonding by the vinyl acetate resin being achieved by evaporating an organic solvent from the preformed mixture containing the sand and resin, thereby achieving the rapid hardening of the binder.
The materials which have heretofore been used as binders in the manufacture of self-curing cores and molds are broadly divided into inorganic binders such as gypsum, cement and sodium silicate and organic binders such as drying oils and cold setting resins, i.e., resins which chemically cure at about ambient temperature. Gypsum and cement are inexpensive, but have the undesirable characteristic of hardening in the presence of water. Thus, the residual water in the sand employed in sand cores and molds can cause the mold to collapse when the water is liberated by the heat of molten metal being cast in the mold. Furthermore, such inorganic binders are deficient in refractoriness and gas permeability.
It is known to employ sodium silicate, in the carbon dioxide mold process where the preformed mixture of sand and binder are cured with carbon dioxide, in the exothermal self-curing mold process which involves mixing of sodium silicate and a powdered metal such as of aluminum, or in the fluid sand self-curing mold process which uses sodium silicate in conjunction with dicalcium silicate and a foaming agent. All these methods are advantageous in that they are inexpensive to use and permit the molds to cure in a short time. Nevertheless, they suffer serious disadvantages such as inferior surface stability, objectionable hygroscopicity, inferior mold collapsibility after casting molten metal and the virtual impossibility of reusing the sand.
Generally use of an inorganic binder entails the serious disadvantage of providing sand molds which are difficult to break or collapse after casting, so that the removal of the cast product from the mold is impeded to a considerable extent.
For the reasons enumerated above, use of self-curing cores and molds employing organic binders has grown.
The method of preparing foundry sand molds using a drying oil-based binder is based on chemical curing of the binder by air and thus is called "the air-setting mold process." This method provides satisfactory mold collapsibility after casting and provides a fine cast surface. Yet, it is disadvantageous in that the setting or curing operation is of long duration and necessitates a heat treatment. This method is also defective in being dependent on the quality of the particular sand used. Cold-setting resins include the urea-(or phenol)-furfuryl alcohol-formaldehyde copolycondensate resins, resol type phenol-formaldehyde resins (or urea copolycondensate resins) and the like. In the cold setting process the cores and molds are chemically cured at ambient temperature by incorporation of a strongly acidic curing agent for the resin such as paratoluene-sulfonic acid or phosphoric acid. In the cold setting process which utilizes an oil-modified alkyd resin and polyisocyanate in the cold box process, a mixture of phenolic resin with an organic isocyanate is molded or preformed to a required shape and the cores and molds are chemically cured by passing an organic amine vapor through the cores and molds.
The sand cores and molds obtained by these methods have greater strength than those obtained by using inorganic binders. They have an additional advantage in that they set rapidly at ambient temperature and provide satisfactory core and mold collapsibility after casting. On the other hand, they have disadvantages of being expensive, or generating irritating or disagreeable odors (such as of amines, formaldehyde or oil) during core-and mold-curing or casting. These methods are disadvantageous in that the quality of the mold is dependent on the quality of the particular sand used and the pot life and setting time of the sand-binder mixtures are difficult to adjust.
Foundry sand binders individually have characteristics of their own, so that the most suitable binder and molding process must be selected with due consideration given to the size and shape of the sand cores or molds required for casting, or the amount of the product desired to be manufactured, etc. Actually there exists no universal binder.